The CPAN Frequently Asked Questions - www.cpan.org

CPAN Frequently Asked Questions

Here are some answers to the most common questions received by cpan@perl.org.

For general information about Perl you should see the Perl Home Page (www.perl.org).

See the Perl FAQ (especially for any Perl programming questions, but also for other resources), available at http://perldoc.perl.org/index-faq.html.

I. - General Questions.

II. - The Quest for Perl source, modules and scripts.

III. - RIF [ Reading is Fun-duh-mental ]

IV. - Danger Will Robinson! Danger! Danger!

V. - Searching CPAN.

VI. - Contributing modules, patches, and bug reports.

VII. - How to mirror CPAN.

What is Perl?

From the Perl documentation:

Perl is a high-level programming language with an eclectic heritage written by Larry Wall and a cast of thousands. It derives from the ubiquitous C programming language and to a lesser extent from sed, awk, the Unix shell, and at least a dozen other tools and languages. Perl's process, file, and text manipulation facilities make it particularly well-suited for tasks involving quick prototyping, system utilities, software tools, system management tasks, database access, graphical programming, networking, and web programming. These strengths make it especially popular with system administrators and web developers, but mathematicians, geneticists, journalists, and even managers also use Perl. Maybe you should, too.

A good starting point for Perl information is http://www.perl.org/

What is Perl6?

Perl and Raku (formerly known as Perl 6) are two languages in the Perl family, but of different lineages. Raku is not intended primarily as a replacement for Perl, but as its own thing - and libraries exist to allow you to call Perl code from Raku programs and vice versa.

For more about Raku see http://www.raku.org/.

"We're really serious about reinventing everything that needs reinventing." --Larry Wall

What is CPAN?

CPAN is the Comprehensive Perl Archive Network, a large collection of Perl software and documentation. You can begin exploring from either http://www.cpan.org/ or any of the mirrors listed at http://www.cpan.org/SITES.html.

CPAN is also the name of a Perl module, CPAN.pm, which is used to download and install Perl software from the CPAN archive. This FAQ covers only a little about the CPAN module and you may find the documentation for it by using perldoc CPAN via the command line or on the web at https://metacpan.org/pod/CPAN.

What is PAUSE?

PAUSE is the Perl Authors Upload SErver, a registry for Perl module, script and documentation authors to upload their work to the CPAN. CPAN and PAUSE are often used interchangeably but they are distinct from each other. The CPAN.pm documentation explains it rather simply;

In this discussion CPAN and PAUSE have become equal -- but they are not. PAUSE is authors/, modules/ and scripts/. CPAN is PAUSE plus the clpa/, doc/, misc/, ports/ and src/.

See the question 'How do I contribute modules?' below if you want to become a registered PAUSE user.

How does the CPAN work?

CPAN works with the generosity and cooperation of thousands of developers, over 1 participating mirrors, many companies, institutions and individuals donating the network bandwidth, storage space and computing power, volunteers who help keep everything together and users whose interest in Perl keep the archive alive and growing.

After an author uploads their module to PAUSE, it will be mirrored to CPAN once an hour and from there, to the rest of the mirrors around the world. There are people who advise authors on their choice of name and namespace for their modules and a few others who answer questions and investigate issues sent to cpan@perl.org.

Where can I find the current release of the Perl source code?

Where can I find older/obsolete versions of Perl or Perl Modules?

Unless you have A Very Good Reason you shouldn't be installing obsolete versions because they might contain bugs, possibly even security bugs.

Good Reasons may include having to support Perl 4 programs, trying to replicate a bug that requires an old Perl release, or pure joy of software archaeology. (Are you Perl 1 compliant?)

CPAN does not carry all ancient releases and patchlevels of Perl (because of the bugs we mentioned above and because they would take quite a lot of storage space).

How do I interpret the Perl version numbers?

Perl changed the version numbering system with v5.6.0 as was indicated in the release announcement:

Perl v5.6.0 is a major release that incorporates all maintenance and development changes since the last major release, 5.005. As you may have noticed, the version numbering has changed. Releases will henceforth be numbered as revision.version.subversion triples. Maintenance releases will have an even version component, while the version component for development releases will be odd. For example, the next maintenance update of Perl 5.6.0 will be v5.6.1, and the development series will begin life at v5.7.0.

You may also peruse the perlhist manpage for a complete list of versions and their release dates.

How do I install Perl using the source code?

To build Perl you need a C compilation environment. After downloading the source code and opening it up, you should first read the INSTALL document which will detail how to build Perl on most systems. There are a number of README.[platform] for platforms where special care is needed in building Perl. As always, reading the documentation is a Good Thing[tm].

Perl can be installed using the standard source code distribution on almost all platforms Perl runs on. This includes all the UNIXes (and good lookalikes, meaning POSIX environments like OS/2, Plan 9, QNX, Amiga, MPE/iX, VMS, OS390, Stratus VOS), and Microsoft platforms. The most notable exceptions are (as of 1999-Mar-24);

  • MacOS (1)
  • AS/400 (2)
  • Novell Netware (2)

For these platforms a binary release may be the easiest path.

  1. The source code to compile MacPerl is available at http://www.cpan.org/ports/mac/.
  2. The source code for AS/400 and Netware Perls have not been merged to the main Perl source code distribution. If you want to try compiling them yourself, get the sources from http://www.cpan.org/ports/As400/ or http://www.cpan.org/ports/netware/ and then continue at http://www.cpan.org/src/README

Where can I find Perl modules?

Due to the ever increasing number of modules on CPAN, the CPAN search engine is possibly a better starting point in your quest for code, especially if you already know exactly what you are looking for.

How do I install Perl modules?

Installing a new module can be as simple as typing perl -MCPAN -e 'install Chocolate::Belgian'. The CPAN.pm documentation has more complete instructions on how to use this convenient tool. If you are uncomfortable with having something take that much control over your software installation, or it otherwise doesn't work for you, the perlmodinstall documentation covers module installation for UNIX, Windows and Macintosh in more familiar terms.

Finally, if you're using ActivePerl on Windows, the PPM (Perl Package Manager) has much of the same functionality as CPAN.pm.

How do I find out what modules are already installed on my system?

  • perldoc perllocal

    Each time a module is installed on your system, it appends information like the following to a file called perllocal.pod which can be found in /usr/local/lib/perl5/version number/architecture/ or something akin to that. The path for your specific installation is in your @INC which you can divine with perl -V.

    =head2 Wed May 12 13:42:53 1999: C<Module> L<Data::Dumper>
    =over 4
    =item *
    C<installed into: /usr/local/lib/perl5/5.00503>
    =item *
    C<LINKTYPE: dynamic>
    =item *
    C<VERSION: 2.101>
    =item *
    C<EXE_FILES: >

    Each entry includes the Module name, date and time it was installed, where it was installed, linktype [ static or dynamic ], version and executables, if any, included with the module.

  • ActivePerl has the PPM utility.
    C:\>ppm query
    Archive-Tar   [0.072 ] module for manipulation of tar archives.
    Compress-Zlib [1.03  ] Interface to zlib compression library
    DBI           [1.13  ] Database independent interface for Perl
    GD            [1.25  ] Interface to Gd Graphics Library
    HTML-Parser   [2.23  ] SGML parser class
    MIME-Base64   [2.11  ] Encoding and decoding of base64 strings
    PPM           [1.1.4 ] Perl Package Manager: locate, install, upgrade software
  • The pmtools suite written by Tom Christiansen to help navigate and manage Perl module installations. You can obtain them at https://metacpan.org/release/pmtools. From the README:
    This is pmtools -- a suite of small programs to help manage modules.
    The names are totally preliminary, and in fact, so is the code.  We follow
    the "keep it small" notion of many tiny tools each doing one thing well,
    eschewing giant megatools with millions of options.
    • pmpath - show the module's full path
    • pmvers - get a module version number
    • pmdesc - get a module description
    • pmall - get all installed modules pmdesc descriptions
    • pmdirs - print the perl module path, newline separated
    • plxload - show what files a given program loads at compile time
    • pmload - show what files a given module loads at compile time
    • pmexp - show a module's exports
    • pminst - find what's installed
    • pmeth - list a class's methods, recursively
    • pmls - long list the module path
    • pmcat - cat the module source through your pager
    • pman - show the module's pod docs
    • pmfunc - show a function source code from a module
    • podgrep - grep in pods of a file
    • pfcat - show pods from perlfunc
    • podtoc - list table of contents of a podpage
    • podpath - show full path of pod file
    • pods - list all standard pods and module pods
    • sitepods - list only pods in site_perl directories
    • basepods - list only normal "man-page" style pods
    • faqpods - list only faq pods
    • modpods - all module pods, including site_perl ones
    • stdpods - list standard pods, not site_perl ones

Where can I find the most recently uploaded Perl modules?

There are so many new and updated modules that it is hard to keep up with the deluge, but there are ways to stay abreast of the tide.

Any of these should be good for your daily feed of new modules.

Where can I find Perl modules for Windows?

www.activestate.com has a FAQ for their Package Manager. Also see http://strawberryperl.com/ which also enables you to build your own modules from CPAN.

Where can I find Perl binaries/packages or Perl module binaries?

http://www.cpan.org/ports/index.html is a current list of Perl binaries that we are aware of at this time. If you have a package for a platform, send us a URL. We do not endorse nor guarantee these packages nor do we store them locally on CPAN due to the potential size of the archive if we did.

Perl module binaries for use with ActivePerl's PPM can be found at http://www.activestate.com/PPMPackages/.

How are Perl and the CPAN modules licensed?

Most, though not all, modules on CPAN are licensed under the GNU Public License (GPL) or the Artistic license and should be stated in the documentation that accompanies the module itself. If the license is not specifically stated in the module, you can always write the author to clarify the issue for you. Also, the text of the Artistic license and the GNU Public License are included in the root directory of the source distribution. From the 'README' file that comes with Perl:

                       Perl Kit, Version 5.0

                   Copyright 1989-1999, Larry Wall
                        All rights reserved.

This program is free software; you can redistribute it and/or modify
it under the terms of either:

    a) the GNU General Public License as published by the Free
    Software Foundation; either version 1, or (at your option) any
    later version, or
    b) the "Artistic License" which comes with this Kit.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
the GNU General Public License or the Artistic License for more details.

You should have received a copy of the Artistic License with this
Kit, in the file named "Artistic".  If not, I'll be glad to provide one.

You should also have received a copy of the GNU General Public License
along with this program in the file named "Copying". If not, write to the
Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307, USA or visit their web page on the internet at

For those of you that choose to use the GNU General Public License,
my interpretation of the GNU General Public License is that no Perl
script falls under the terms of the GPL unless you explicitly put
said script under the terms of the GPL yourself.  Furthermore, any
object code linked with perl does not automatically fall under the
terms of the GPL, provided such object code only adds definitions
of subroutines and variables, and does not otherwise impair the
resulting interpreter from executing any standard Perl script.  I
consider linking in C subroutines in this manner to be the moral
equivalent of defining subroutines in the Perl language itself.  You
may sell such an object file as proprietary provided that you provide
or offer to provide the Perl source, as specified by the GNU General
Public License.  (This is merely an alternate way of specifying input
to the program.)  You may also sell a binary produced by the dumping of
a running Perl script that belongs to you, provided that you provide or
offer to provide the Perl source as specified by the GPL.  (The
fact that a Perl interpreter and your code are in the same binary file
is, in this case, a form of mere aggregation.)  This is my interpretation
of the GPL.  If you still have concerns or difficulties understanding
my intent, feel free to contact me.  Of course, the Artistic License
spells all this out for your protection, so you may prefer to use that.

Does the Perl source include any modules?

Yes, Perl comes with a number of useful modules and are listed in the perlmodlib pod:

Where can I find Perl scripts?

Where can I find the Perl FAQs?

The Perl FAQ is included with the Perl source code distribution.

Where can I find Perl documentation?

Where can I find Perl module documentation?

Where do I find Perl DBI/DBD/database documentation?

Where can I find Perl mailing lists?

There are quite a few mailing lists with a broad range of topics.

Where can I find Perl courses/training/on-line tutorials?


How do I find/join/organise a Perl User Group?

The Perl User Groups are known as "Perl Mongers" and have active groups all over the world. You can find an established group at http://www.pm.org/groups/ or start a new group if one isn't near you via http://www.pm.org/start/

Where can I find a history of Perl releases or a general history of the Perl community?

A history of Perl releases can be found in your Perl distribution via perldoc perlhist or via the web at http://perldoc.perl.org/perlhist.html. A more general history of the Perl community, CPAST, can be found at http://history.perl.org/.

I downloaded a module/script/file but it was corrupt, what should I do?

Many CPAN filenames end in .tar.gz. Unfortunately some programs mutilate such names (e.g., rename them with _tar.tar) so that unpacking programs don't recognise them and refuse to unpack them. Try saving the file using the .tgz suffix or try changing your web client. Also, you could try a plain FTP client as almost all the CPAN sites are ftp-reachable. You can find the full list of mirrors http://www.cpan.org/SITES.html

If you use FTP remember to download in binary format, not text format.

Please read http://www.cpan.org/ENDINGS if you aren't sure what the files should be unpacked with and want to know if you are using the right program.

If you still think you have a corrupt file, try downloading the file from another site. If you still have no satisfaction, then please let us know the exact file name and URL/FTP site and path.

How do I use module Foo::Bar, can you help? (a.k.a. Are you a helpdesk?)

We at CPAN are not a helpdesk. We may point you towards a plethora of documentation to help you in your quest for knowledge but we cannot debug your code or read for you. We exist specifically to answer questions and solve problems relating directly to the functioning of the CPAN itself.

In addition to the on-line documentation you might try the newsgroup comp.lang.perl.modules for help with a particular module. Also, looking at other code using the same module might prove enlightening.

How do I search for anything in CPAN?

By using a CPAN search engine.

  • https://metacpan.org/ - A front-end for api.metacpan.org, which was created so that CPAN metadata can be searched and accessed without necessarily having to use any specific interface.
  • http://perldoc.perl.org/ - also useful for core perl documentation.

How do I search for module/script documentation? (also known as "How do I use ..."?)

In general modules and scripts come with their own documentation which should have been installed along with your module/script. (Thanks to Perl's pod-style documentation, "it is very hard to misplace your documentation".)

How do I contribute modules to CPAN?

If you would like to learn more about PAUSE and how to go about contributing your module to CPAN please read the PAUSE FAQ at http://www.cpan.org/modules/04pause.html which will tell you how to go about getting a PAUSE ID and the steps needed to upload your code. Also,perldoc perlmodlib and perldoc perlmod are a good introduction to Perl modules.

Does CPAN allow contributions of shareware or code that requests a fee of any kind?

No. Everything on CPAN is free of charge. The reason for this is that CPAN is the product of hundreds of people donating their time and resources for the common good of the Perl community. There are places on the net where one can offer shareware without treading on the generosity of others and this is not that place.

How do I contribute scripts to CPAN?

Scripts can be provided in any CPAN distribution, though distributions need to contain a module to be easily installable. By convention, CPAN modules meant to install scripts are usually put in the App:: namespace.

How do I contribute documentation to CPAN?

If the documentation is for a particular module that isn't a core distribution module, then please send it to the module author. If the module is a core module the most appropriate place to send doc patches and enhancements is the Perl5Porters mailing list.

How do I report/fix a bug in Perl and/or its documentation?

Always remember to make your bug reports as detailed as possible. "Perl doesn't work." is not a bug report.
Please note that problems concerning modules that are installed separately from the Perl distribution (such as Tk) are reported differently.

Here is a checklist from perlbug, a bug reporting tool included in your Perl distribution. It is a bit on the long side, but please read it carefully as the better your bug report is, the more likely the issue will be addressed.

  • What version of Perl you are running?

    Try perl -v at the command line to find out.

  • Are you running the latest released version of perl?

    Look at http://www.perl.com/ to find out. If it is not the latest released version, get that one and see whether your bug has been fixed. Note that bug reports about old versions of Perl, especially those prior to the 5.0 release, are less likely to be incorporated into the source as Perl1 through Perl4 are largely unmaintained.

  • Are you sure what you have is a bug?

    A significant number of the bug reports we get turn out to be documented features in Perl. Make sure the behavior you are witnessing doesn't fall under that category, by glancing through the documentation that comes with Perl (we'll admit this is no small task, given the sheer volume of it all, but at least have a look at the sections that seem relevant).

    Be aware of the familiar traps that perl programmers of various hues fall into. See the perltrap documentation.

    Check in perldiag to see what any Perl error message(s) mean. If message isn't in perldiag, it probably isn't generated by Perl. Consult your operating system documentation instead.

    If you are on a non-UNIX platform check also perlport documentation, some features may not be implemented or work differently.

    Try to study the problem under the Perl debugger,if necessary. See the perldebug documentation.

  • Do you have a proper test case?

    The easier it is to reproduce your bug, the more likely it will be fixed, because if no one can duplicate the problem, no one can fix it. A good test case has most of these attributes: fewest possible number of lines; few dependencies on external commands, modules, or libraries; runs on most platforms unimpeded; and is self-documenting.

    A good test case is almost always a good candidate to be on the perl test suite. If you have the time, consider making your test case so that it will readily fit into the standard test suite.

    Remember also to include all the exact error messages, if any. "Perl complained something" is not an exact error message.

    If you get a core dump (or equivalent), you may use a debugger (dbx, gdb, etc) to produce a stack trace to include in the bug report. NOTE: unless your Perl has been compiled with debug info (often -g), the stack trace is likely to be somewhat hard to use because it will most probably contain only the function names and not their arguments. If possible, recompile your Perl with debug info and reproduce the dump and the stack trace.

  • Can you describe the bug in plain English?

    The easier it is to understand a reproducible bug, the more likely it will be fixed. Anything you can provide by way of insight into the problem helps a great deal. In other words, try to analyze the problem (to the extent you can) and report your discoveries.

  • Can you fix the bug yourself?

    A bug report which includes a patch to fix it will almost definitely be fixed. Use the diff program to generate your patches (diff is being maintained by the GNU folks as part of the diffutils package, so you should be able to get it from any of the GNU software repositories). If you do submit a patch, the cool-dude counter at perlbug@perl.com will register you as a savior of the world. Your patch may be returned with requests for changes, or requests for more detailed explanations about your fix.

    Here are some clues for creating quality patches:
    Use the -c or -u switches to the diff program (to create a so-called context or unified diff). Make sure the patch is not reversed (the first argument to diff is typically the original file, the second argument your changed file). Make sure you test your patch by applying it with the patch program before you send it on its way. Try to follow the same style as the code you are trying to patch. Make sure your patch really does work (make test, if the thing you're patching supports it).

  • Can you use perlbug to submit the report?

    perlbug will, amongst other things, ensure your report includes crucial information about your version of perl. If perlbug is unable to mail your report after you have typed it in, you may have to compose the message yourself, add the output produced by perlbug -d and email it to perlbug@perl.com. If, for some reason, you cannot run perlbug at all on your system, be sure to include the entire output produced by running perl -V (note the uppercase V).

    Whether you use perlbug or send the email manually, please make your Subject line informative. "a bug" is not informative. Neither is "perl crashes" nor "HELP!!!" These don't help. A compact description of what's wrong is fine.

    Having done your bit, please be prepared to wait, to be told the bug is in your code, or even to get no reply at all. The Perl maintainers are busy folks, so if your problem is a small one or if it is difficult to understand or already known, they may not respond with a personal reply. If it is important to you that your bug be fixed, do monitor the Changes file in any development releases since the time you submitted the bug, and encourage the maintainers with kind words (but never any flames!). Feel free to resend your bug report if the next released version of perl comes out and your bug is still present.

How do I report/fix a bug in a module/script?

Please contact the author of the module/script, ideally by creating a ticket using the bug tracker for the module (linked as 'Bugs' from metacpan.org). The author should be automatically notified by email.

If the author doesn't respond, the documentation of the module/script might contain a contact address or you can try CPANID@perl.org where CPANID is the authors CPANID.

Most of the checklist in reporting bugs in Perl above applies for modules as well. Make your bug report as good as possible if you really want the bug fixed. If the module is included with the Perl distribution you should also follow the Perl bug reporting tips.

How do I go about maintaining a module when the author is unresponsive?

Sometimes a module goes unmaintained for a while due to the author pursuing other interests, being busy, etc. and another person needs changes applied to that module and may become frustrated when their bug reports and emails goes unanswered. CPAN does not mediate or dictate a policy in this situation and rely on the respective authors to work out the details. If you treat other authors as you would like to be treated in the same situation the manner in which you go about dealing with such problems should be obvious.

  • Be courteous.
  • Be considerate.
  • Make an earnest attempt to contact the author as explained in How do I report/fix a bug in a module/script?.
  • Give it time. If you need changes made immediately, consider applying your patches to the current module, changing the version and requiring that version for your application. Eventually the author will turn up and apply your patches, offer you maintenance of the module or, if the author doesn't respond in a year, you may get maintenance by having interest.
  • If you need changes in order for another module or application to work, consider making the needed changes and bundling the new version with your own distribution and noting the change well in the documentation. Do not upload the new version under the same namespace to CPAN until the matter has been resolved with the author or CPAN.

Simply keep in mind that you are dealing with a person who invested time and care into something. A little respect and courtesy go a long way.

How do I adopt or take over a module already on CPAN?

The easiest way to take over a module is to have the current module maintainer either make you a co-maintainer or transfer the module to you.

If you can't reach the author for some reason (e.g. email bounces), the PAUSE admins at modules@perl.org can help. The PAUSE admins treat each case individually.

Please make sure you have tried all of the above ways of getting in contact with the author before going this way!

  • Get a login for the Perl Authors Upload Server (PAUSE) if you don't already have one: http://pause.perl.org
  • Write to modules@perl.org explaining what you did to contact the current maintainer including all email addresses of the author in cc and links to the tickets you have opened. The PAUSE admins will also try to reach the maintainer.
  • Post a public message in a heavily trafficked site announcing your intention to take over the module. Potential places are http://blogs.perl.org/, http://www.perlmonks.org/, and any appropriate mailing lists or web forums.
  • Wait a bit. The PAUSE admins don't want to act too quickly in case the current maintainer is on holiday. If there's no response to private communication or the public post, a PAUSE admin can transfer it to you.

Is there a site for module bug reports/tests?

Yes, through the diligence of Paul Schinder and a few others, we have CPAN Testers which is a collection of test results for modules on a number of different platforms. This information is also available when viewing module information on Metacpan.

There is also http://bugs.perl.org/ where you might search for a module bug already reported to P5P.

Does CPAN provide download statistics for authors?

No we don't. http://xxx.lanl.gov/help/faq/statfaq sums up our thoughts on the matter quite well.

Questions about mirroring CPAN

See How to mirror CPAN.

The CPAN FAQ is copyright Jarkko Hietaniemi and Elaine Ashton 1998-2007 and Ask Bjørn Hansen, Leo Lapworth 2011 All Rights Reserved. Send questions and comments to cpan@perl.org.